This invention relates to viewing screens for projected light beam display systems and, more particularly, to a viewing screen which provides increased brightness within a defined observation pupil.
The viewing, or observation, pupil of a projection screen is the locus of all locations from which the image projected upon the screen may be seen, i.e., the image may be seen when the observer's eye is within the pupil, and not otherwise. There are a number of reasons why it may be desirable to control the shape and size of the observation pupil. For example, for purposes of energy conservation, it would be desirable to maximize the image brightness for observers located within the proper viewing area. It may also be desirable, for certain applications, to prevent those outside the desired viewing area from seeing the image projected on the screen.
There have been various attempts in the past to satisfy these objectives. For example, a special screen has been proposed for drive-in movie theaters where it is desired that none of the images projected on the screen can be seen outside of the paid admission area. This particular screen is a lenticular array made up of half inch square curved reflectors that reflect the incident light only into the desired area. Another proposed technique introduces a small amount of lubricant between two sheets of smooth aluminum which are then rolled between metal rollers under high pressure. The result is a large number of depressions which tend to act like randomly distributed retro-reflectors. Yet another technique is to arrange an array of lenslets in a linear, periodic pattern, backed by a specular or diffuse reflector. Still another technique is to use a large number of tiny glass beads which are adhered to a surface. With the proper index of refraction of the beads, the rear surface coincides with the focal sphere of the front surface, thus forming an autocollimating, or "cat's eye", configuration. Still another technique is to provide an array of corner reflectors arranged on a panel. This screen is also retroreflective since each corner reflector is retrodirective.
None of the aforedescribed attempts have proven entirely satisfactory because they do not achieve a uniform gain profile over the observation pupil, and further, except for the drive-in movie screen all of the resulting screens are retroreflecting, i.e., the light is reflected directly back into the projector with greatest intensity. In order to eliminate this problem, a specular reflector must be provided behind each element of the screen to direct the light in the desired direction. This presents a tremendous complication to manufacturing a screen. It is therefore an object of this invention to provide an improved projection viewing screen wherein the observation pupil may be precisely defined.